In transport networks, WDM technology particularly DWDM technology offers many benefits in terms of bandwidth capabilities and scalability. Such networks can be passive (PON) or active. WDM-PON brings this benefit most notably in access networks, offering high capacity (currently upgradeable to 10 Gb/s), long distances, no bandwidth contention (virtual point-to-point) and service transparency, together with the possibility of smooth upgrades (per channel) in the protocol and in the bit-rate. WDM-PON is an emerging technology also for mobile backhaul, since broadband services and bandwidth demands are quickly increasing. A conventional WDM-PON is realized with a tree topology with a passive AWG at the remote node (RN) acting as a distribution node for mux/demux of the channels. This topology supports a high number of ONTs with the same kind of ONT for any AWG port (colorless).
It is known to use different wavelengths for the upstream and downstream signals. To increase system capacity, it is possible to use the same wavelengths for both directions, known as wavelength reuse, but this can give rise to optical cross talk interference, depending partly on the relative optical powers of the signals.
It is also known to provide WDM PONs with bidirectional optical amplification to enable an increase in the optical reach, or in the number of optical drops on each channel.
Bidirectional optical amplification is well known to be critical, due to the RIN generated by multiple reflections of amplified light, triggered by lumped reflections or Rayleigh Backscattering (RBS) in fiber (as explained in “Estimation of Performance Degradation of Bidirectional WDM Transmission System Due to Rayleigh Backscattering and ASE Noise Using Numerical and Analitical Models” J. Ko, S. Kim, J. Lee, S. Won, Y. S. Kim, J. Jeong. Journal of Lightwave Technology, Vol. 21, No 4, April 2003). This is why it is advisable to separately amplify the two propagation directions, e.g. by using optical circulators, even in bidirectional links.
Also in this case, however, interferometric cross-talk can arise between downstream and upstream channels due to bad isolation of optical components or, again, optical reflections in the link. This explains why any WDM system based on wavelength reuse proposed so far is passive. Nevertheless, optical amplification could significantly expand the application range of those systems, e.g. covering network scenarios where the average distance between user and central office is increased following sites consolidation or for service upgrade of deployed PONs with passive splitters.
Several wavelength reuse techniques have been proposed to realize low-cost wavelength agnostic (=colorless) WDM interfaces in bidirectional optical links, e.g. saturated or feed-forward controlled RSOAs or remodulation schemes such as FSK/ASK, IRZ/RZ, Manchester or 8B10B/NRZ, etc. All these techniques mitigate the cross-talk between opposite propagation directions but in the presence of optical amplification they may suffer from an unacceptable increase the power level of interfering light.